1. Field of the Invention
Enhancements to a video anticopy process, the enhancements causing additional degradation to the picture quality when a copy of a protected recording is played back, and additionally which reduce the viewability of unauthorized recordings of the protected recording.
2. Description of the Prior Art
Video anticopy processes are well known. An example is Ryan, U.S. Pat. No. 4,631,603 issued Dec. 23, 1986, incorporated by reference which discloses (see Abstract):
xe2x80x9cA video signal is modified so that a television receiver will still provide a normal color picture from the modified video signal while the video tape recording of the modified video signal produces generally unacceptable pictures. This invention relies on the fact that typical videocassette recorder automatic gain control systems cannot distinguish between the normal sync pulses (including equalizing or broad pulses) of a conventional video signal and added pseudo-sync pulses. Pseudo-sync pulses are defined here as any other pulses which extend down to normal sync tip level and which have a duration of at least 0.5 microseconds. A plurality of such pseudo-sync pulses is added to the conventional video signal during the vertical blanking interval, and each of said pseudo-sync pulses is followed by a positive pulse of suitable amplitude and duration. As a result, the automatic gain control system in a videotape recorder will make a false measurement of video level which causes an improper recording of the video signal. The result is unacceptable picture quality during playback.xe2x80x9d
Column 2, beginning at line 5 states that the added pulse pairs (each pair being a negative-going pseudo-sync pulse followed by a positive-going xe2x80x9cAGCxe2x80x9d pulse) cause an automatic level (gain) control circuit in a videotape recorder to erroneously sense video signal level and produce a gain correction that results in an unacceptable videotape recording.
Therefore this prior art xe2x80x9cbasic anticopy processxe2x80x9d causes an abnormally low amplitude video signal to be recorded when a copy is attempted. Some of the effects observed when the illegal copy is replayed are horizontal tearing (positional displacement) and vertical displacement of the picture. Whether this occurs or not is often largely dependent on the picture content, i.e. presence of white (light) and black (dark) areas in the picture. Therefore this prior art process, while generally providing excellent copy protection, with some combinations of videotape recorders (such as VCRs) and television sets provides a picture viewable by persons willing to tolerate a poor quality picture.
Also, with certain VCRs and TV sets the various well known prior art copy protection processes provide little picture degradation. Certain markets for prerecorded video material have a high rate of piracy, i.e. illegal copying of videotapes, in spite of copy protection and these viewers apparently are relatively insensitive to the poor quality picture in illegal copies caused by the prior art copy protection processes. Thus there is a need for copy protection process enhancements which degrade the quality of the picture even more than that of the prior art processes.
In accordance with the present invention, the above-described prior art xe2x80x9cbasicxe2x80x9d copy protection process is enhanced by further modifying the video signal in several ways to ensure that the necessary picture content requirement is met to maximize effectiveness of the basic copy protection process.
The further modifications include blanking a portion of the active video in the overscan area of the picture just prior to the occurrence of the (1) horizontal or (2) vertical synchronization (sync) signals, and inserting into the blanked portion a waveform that (for a video signal having reduced amplitude) is perceived by the TV receiver or videotape recorder as a sync signal, and so causes incorrect synchronization of the VCR or TV receiver. Using this modification especially only on certain video lines or fields causes substantial picture degradation of an unauthorized copy. Another modification narrows the horizontal sync pulses to cause sensing of a spurious vertical sync signal in a TV set and will also affect certain videotape recorders.
In the horizontal modification, the right edge of the picture is replaced by a xe2x80x9ccheckerxe2x80x9d pattern appearing (like a checker board) of black and gray rectangles. The width of this checker pattern is chosen to be within the overscan (not viewed) part of the picture when displayed on a standard television receiver. It will be understood that with an abnormally low signal amplitude, when the picture content is light (such as mid gray), the left edge of the black rectangle in certain video lines will trigger an early horizontal retrace as being a negative-going (towards blanking level) transition. When the picture content is dark, the right edge of the gray rectangle (adjacent to a dark picture area), in certain video lines will trigger an early retrace on each line as also being a negative-going transition. (The description of video waveforms herein follows the convention of positive amplitude being white and negative amplitude being black).
The horizontal modification checker pattern in one embodiment is generated at a rate slightly asynchronous to the video field repetition rate, so that the checker pattern appears to slowly move up or down the picture, at a rate of about 1 second for any given point to migrate from the bottom to the top of the picture or vice versa. The checker pattern has no effect on the picture when an original (authorized) cassette is replayed since no signal conditions are present in the TV set which are in any way abnormal.
However, when an illegal (unauthorized or pirated) copy of the cassette is replayed using a videotape recorder, the signal attenuation resulting from the above-described prior art copy protection process, in combination with the checker pattern, causes the television set horizontal retrace to occur early in each video line where either when the black or gray rectangle is present, depending on picture content and the characteristics of the videotape recorder and TV set. The black checkers and gray checkers each may cause a transition of sufficient amplitude depending on the previous active video picture content. If the picture content is light (white), the left edge of the black checker causes a negative-going transition to black; if the picture content is dark, the right edge of the gray checker causes a negative-going transition from gray to the following dark area (typically blanking level). The difference between lines ending in black or gray in turn causes a horizontal displacement to the picture information, i.e. a wiggle, which moves slowly up or down the picture.
The tendency of a television set to retrace (perform the horizontal flyback early) is exploited by providing the light to dark transition (the left edge of the black checker or the right edge of the gray checker) prior to the location in the video line of the genuine horizontal line synchronization (sync) signal. The early retrace so triggered causes the picture information on the succeeding line to be advanced, i.e. displaced horizontally to the right by an amount equal to the distance between the negative transition and the location of the leading edge of the genuine horizontal sync signal. This displacement causes a xe2x80x9ctearingxe2x80x9d (horizontal repositioning) of picture information.
A somewhat similar modification in the vertical picture sense inserts alternating dark and white bands in place of active video in the last few lines of selected video fields in the lower overscan portion of the picture just prior to the vertical blanking interval, and/or extending into the first few lines of the vertical blanking interval.
This vertical rate modification is implemented in several ways. In one embodiment several of the active video lines (five or so) immediately prior to the vertical sync signal are made to alternate between blanking level and a gray level (typically about 30% of peak white) at a rate of about 1 to 5 cycles per second. This can cause drum servo unlock in the copying videotape recorder, or erroneous vertical retrace in the TV set, causing the picture from the unauthorized copy to exhibit vertical instability (jump up and down) at that particular rate, substantially degrading the quality of the image. In another version, two to five lines of alternating (modulated) white-black-white are inserted at the end of each or alternate video fields, with the same result of loss of vertical lock in a copying videotape recorder or viewing TV set due to interpretation of the inserted pattern as a vertical sync signal when the video signal amplitude has been reduced through AGC response to a copy protection signal.
These vertical modifications in another version are both extended into the first few lines of the subsequent vertical blanking interval.
Addition of pulses to portions of the video signal after normal horizontal or video synchronization pulses cause an abnormal video retrace at this point, thereby being an effective enhancement to the prior art basic anticopy process. Typically these added post-vertical synchronization pulses are at e.g. lines 22-24 of an NTSC television signal.
Thus, the processes in accordance with the invention ensure optimum conditions in terms of picture content for causing the maximum level of subjective degradation (1) to the replayed picture quality of the unauthorized copy and (2) to the recording and playback functions of videotape recorders.
The television set in response to the horizontal and vertical modifications erroneously performs the horizontal or vertical retrace at an abnormal point. In the same way that a TV set will misinterpret the signal, both the recording videotape recorder when the copy is made or the playback videotape recorder when the copy is replayed can also be affected. In this case it is the color circuitry of the videotape recorder which is affected, with resultant picture degradation additional to that caused by the basic anticopy process. This is an additional effect to what has been described so far. This is because of the special way a videotape recorder processes the color information. The picture distortions include inaccurate color rendition and intermittent or permanent loss of color. The objective of the modifications thus is to further destroy the entertainment value of the illegal copy, over and above the degradation of the picture quality caused by the above-described basic prior art copy protection process.
The third modification to the video signal involves narrowing horizontal sync pulses. In combination with a copy protected video signal having reduced signal amplitude when re-recorded (copied), this narrowing causes the sensing of spurious vertical sync signals by a videotape recorder or TV set, causing vertical retrace to take place at other than the beginning of a field and so further degrading picture quality. This modification narrows the width (duration) of the horizontal sync pulses on certain lines (such as lines 250-262) of the video field. These narrowed horizontal sync pulses, when combined with a video signal that is of diminished amplitude, trigger a spurious vertical retrace in many TV sets and videotape recorders, further degrading the displayed picture. Narrowing the horizontal sync pulses where the checker patterns exist (lines 10-250) also enhances the checker pattern distortion when an illegal copy is made.
It has been observed that the degradation of the picture quality in accordance with the present invention is particularly useful where the prior art basic copy protection process provides relatively small degradation of picture quality or relatively small degradation of videotape recorder recording or playback. Thus the combination of the prior art process and the present processes severely reduces entertainment value of the illegal copy on a much larger combination of videotape recorders and TV receivers than does the basic prior art process by itself.
Provision of the horizontal checker pattern or vertical modification only in the overscan portions of the television picture ensures that when the original recording or signal is viewed there is no visibility of the checker pattern or vertical modification, and indeed the presence thereof is not known to the viewer of the original recording.
In other embodiments, the process user might trade off picture area for effectiveness. (The user may elect to trade off visibility of the process when the xe2x80x9clegalxe2x80x9d recording is played, in order to enhance the process anticopy effectiveness.) Thus, the modifications may in violation of broadcast TV standards extend into the viewable portion of the video field, but still be acceptable in many applications. Furthermore, in another embodiment, the process trades off deviations from accepted signal standards to further enhance the anticopy effectiveness.
The modified signal in any case is displayed normally on any TV receiver or monitor, providing the signal is of the correct amplitude. When the modified signal amplitude is reduced, as on an illegal copy, the conditions are optimized for the TV receiver to display or for a videotape recorder to playback a distorted picture. This will occur in a back-to-back video tape recorder copying situation using two videotape recorders when the recording being (illegally) copied is provided with the basic anticopy process of the above-referenced U.S. Pat. No. 4,631,603.
The video signal modifications in accordance with the invention, in addition to causing lack of horizontal or vertical stability in a TV receiver, also additionally have similar effects as described above on a typical videocassette (videotape) recorder, both during recording and playback. VCRs use the leading edge of the horizontal sync pulse to correctly position the burst gate. If the burst gate is incorrectly positioned, the color burst is not be sampled properly and loss of color or distorted color results. The horizontal modification causes misinterpretation of the position of the leading edge of horizontal sync. This will occur in the VCRs involved in both the recording and playback of a (copy protected) copy, resulting in color loss/distortion. This effect can also be caused independently in the TV set. In the same way that a TV set will tend to lose vertical lock as a result of this process, so will a VCR. The result is a loss of drum servo lock in the VCR.
The modifications disclosed herein ensure that the required conditions for maximum picture disruption are always present, rather than relying on chance (the particular picture being displayed) that these conditions occur. Therefore the above processes which may include the horizontal and/or vertical modifications and/or horizontal sync pulse narrowing have substantial value in enhancing the above-described basic prior art copy protection process, and more generally enhance any copy protection process which reduces the amplitude of the video signal which is recorded when an unauthorized copy is attempted. Another embodiment to enhance horizontal jitter with illegal copying of video tapes is to use post horizontal pseudo sync pulses of approximately xe2x88x9220 IRE amplitude (xe2x88x9240 IRE equals normal sync amplitude) and a width of about 1-2 xcexcs varying in position in about a range of 1-2 xcexcs after color burst.
While the embodiments disclosed herein are in the context of the NTSC television standard, with modifications apparent to one of ordinary skill in the art they are applicable to SECAM or PAL television standard.
Also disclosed herein in accordance with the invention are several methods and apparatuses for removal or xe2x80x9cdefeatxe2x80x9d of the above-described video signal modifications, to permit unhampered copying and viewing thereof. The defeat method and apparatus in one version replace or level shift the vertical and horizontal modification pulses with a fixed level gray signal, and defeat the sync pulse narrowing modification by sync widening or replacement.
In other versions, the defeat method uses added pre-horizontal sync pulses, post-horizontal sync pulses, or attenuation averaging. Also disclosed is a new method of defeating the prior art basic video anticopying process.